The operating properties of the refractive-type optical limiting devices based on the self-focusing and defocusing phenomena are studied numerically, and the effects of both the installation position of the nonlinear refractive medium and the waist size of the incident Gaussian beam on the operating performance of the refractive-type optical limiters are examined. Our results show that the optical limiting curves of the refractive-type optical limiters exhibit a slowly attenuating oscillating waveform. The optical limiting threshold and the clamped output value of the optical limiters are dependant on the installation position of the sample and the convergence of the incident Gaussian beam to a great extent. When the sample position is given, the smaller the beam-waist radius, the better the optical limiting performance. When the beam-waist radius of the incident light is kept constant, an optimal position of the nonlinear sample can be found in the system. With fluctuating beam-waist size, however, the optimal location varies within a small range. For a self-focusing (self-defocusing) optical limiter, the optimal optical limiting effect can be obtained when the nonlinear medium is positioned at a certain location within half a Rayleigh range in the left (right) side of the focal plane.
The optimal matching problem of the nonlinear refractive phase-shift and the nonlinear absorptive phase-shift in optical limiters is studied by means of the Gaussian beam Z-scan diffraction theoretical model. Optical limiting behaviors in the limits of the small nonlinear absorption and the large nonlinear absorption are examined. The Gaussian beam Z-scan diffraction theoretical mode and the method of simulating the optical limiting characteristics are presented, and typical numerical computational findings are given. Our results show that, when the nonlinear absorption of an optical limiting material is negligible, the optical limiting characteristic curve exhibits a kind of slowly attenuating oscillating waveforms. In this case, the optical limiting threshold and the clamped output value of the optical limiter are dependent on the installation position of the sample and the convergence of the incident Gaussian beam. When the phase-shift produced by the nonlinear refraction is comparable with that by the nonlinear absorption, the two optical limiting effects from the nonlinear refraction and the nonlinear absorption cancel out to some extent. Our results also show that the ratio of the nonlinear absorptive phase-shift to the nonlinear refractive phase-shift has a great impact on the optical power entering the far-field receiving aperture. To acquire a satisfactory optical limiting performance, a large enough difference must exist between the nonlinear absorptive phase-shift and the nonlinear refractive phase-shift.
This paper investigates an optical reorientation process of azo-dye doped liquid crystals and explains the dependence of the enhancement factor on the incident light direction. Because the order parameters of the guest-host liquid crystal system depend on the direction of the incident light, the intermolecular orientation interaction potentials is also related with the incident light direction. To describe the interaction between the cis-isomer and the liquid crystalline molecules, we re-wrote Maier-Saupe potential expression by means of the order parameters of the doped liquid crystal system. Beginning with a two-level coupling equation, we investigated a microscopic formula and an angular dependence of the enhancement factor with the liquid crystal E63 doped with the R4 azo-dye, and obtained the relation of the enhancement factor with the rotational diffusion constants and the host order parameter. The comparison of our computational results with the existent experimental results verified our enhancement factor's microscopic form.
We study numerically the optical limiting properties of the refractive optical limiting devices based on the self-focusing and defocusing phenomena, and examine the effects of the installation position of the nonlinear media and the waist size of the incident Gaussian beam on the operating performances of the refractive optical limiters. Our results show that the optical limiting curves of the refractive optical limiters assume a vibrating decaying waveform. The optical limiting threshold and the clamped output value of the optical limiters are dependant on the installation position of the sample and the convergence of the incident Gaussian beam to a great extent. When the sample position is given, the smaller the beam-waist radius, the better the optical limiting performance. When the waist radius of the incident light keeps constant, an optimum location of the nonlinear sample can be found in the system. With the fluctuation of the waist size, however, the optimum location changes within a small range. For a self-focusing (self-defocusing) optical limiter, the optimum optical limiting effect can be obtained when the nonlinear medium is positioned at a certain location within half a Rayleigh range in the left (right) side of the focal plane.
The optical limiting characteristics of the methyl-red dye doped nematic liquid crystal (NLC) films have been tested, and an interesting dark spot phenomenon and a dynamic self-action diffraction ring pattern in the far field behind the sample cell have been observed. The time sequence of the dark spot formation and the dynamic self-action diffractino ring apperaance has also been investigated. We found that, with increasing the energy impinging on the sample cell, the dark spot accompanied by a single outer bright ring always appears and lasts until the multiple diffraction rings form. We also found that the dark spot with a single outer bright ring in the far field corresponds to the nonlinear section of the optical limiting characteristic curves of the doped NLC fimls, while the onset of the multiple diffraction rings corresponds to the input damage threshold of the doped NLC film limiters. Our experimental results show that the optical limiter based on the methyl-red dye doped NLC films has a quite low optical limiting threshold and an unusually low clamped output to eye safety. In this paper, we will present how we tested the optical limiting characteristics of the methyl-red dye doped NLC films and what kind of the typical test results we obtained in the experiments. We will theoretically analyze the optical limiting mechanisms of the methyl-red dye doped NLC films. We will also discuss the advantages and some limitations of the methyl-red dye doped NLC films. We will also discuss the advantages and some limitations of the methyl-red dye liquid crystal limiters, and propose several possible solutions to these limitations.
The purpose of this paper is to present a grating-assisted nonlinear limiting method based on the principle of index mismatching and grating diffraction. In addition, various nonlinear organic materials have been tested in the optical limiting system. A few examples with experimental results are presented. Using a YAG nanosecond pulse laser at 532 nm as a simulated threat laser source, we tested the optical limiting characteristics of the methyl-red doped nematic liquid crystal films. In the meantime, we observed some quite interesting optical phenomena, such as self-defocusing diffraction rings, dark spots in the far-field behind the cell. Our experimental results presented in the paper show that the methyl-red doped nematic liquid crystal films posses a good optical limiting performance. When the input energy focused on the cell increases from approximately zero micro Joule to approximately 200 micro Joule, the output energy on the far-field exit plane is kept under 1 micro Joule. We will describe a completed optical limiting system with Q-switched frequency-doubled 12- ns Nd:YAG laser, dual-channel detectors, and computer processing units based on Lab-view software and its design considerations.
The problem of correlation wavefront sensing using an extended arbitrarily structured target as a light source is studied. The principle of extended target correlation wavefront sensing employing an intensity-transmittance type of mask is analyzed. The problems of simulation and selection of a target in laboratory experiments are discussed. The methods of generating the mask images from the target structure and of preparing the experimental masks from the mask images are presented. Typical results of the mask images generated by processing the target image are given. The preliminary experiment on the extended target wavefront sensing using the intensity-transmittance type mask are made. The effects of two different intensity-type masks on wavefront sensing are compared. Experimental results that are in agreement with the theory are obtained.
The correlation wavefront sensing technique using extended, arbitrarily structured target as light source is studied. The principle of extended target correlation wavefront sensing employing an intensity-type mask is analyzed. The problems of simulation and selection of target in laboratory experiments are discussed. The methods of generating the mask images from target structure and of preparing the experimental masks from the mask images are presented. The typical results of the mask images generated by processing the target image are given. A first experiment on the extended target wavefront sensing using the intensity-type mask is made. The effects of two different intensity-type masks on wavefront sensing are compared. The experimental results in agreement with the theory are obtained.
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